Retinoic acid (RA) and its analogs have strong anti-cancer efficacy in human cancers, owing to their well-known differentiation and apoptosis-inducing abilities. However, RA is a liver mitogen in vivo. While there has been speculation that RA-mediated signaling likely is involved in hepatocyte proliferation during liver regeneration, direct evidence is lacking. Findings in support of this proposition include the earlier observation that there was a release of retinyl palmitate (the precursor of RA) from liver stellate cells following liver injury. However, it is virtually unknown regarding the biological action of the released vitamin A. It was speculated that the released vitamin A is converted to RA, the biological form, and then bound to its specific receptor (retinoid x receptor; RXRa), which is abundantly expressed in liver. Consider the mitogenic effect of RA, the activated RXRa would likely then influence hepatocyte proliferation and liver tissue repair. Our study showed that there was a marked delay in cell proliferation in the RXRa-deficient regenerating liver, which provides the first direct evidence indicating the importance of retinoid-RXRa signaling pathways in liver regeneration. However, the mechanism by which RA stimulates hepatocyte proliferation is largely unknown at present. The overall objective of this project, therefore, is to elucidate the mechanism(s) underlying the hepatocyte RXRa- mediated signaling pathways that control hepatocyte proliferation and liver regeneration. We will focus on the pathways that can be activated by RA. Based on our preliminary data, particular emphasis is placed on RXRa dimers formed with Nur77 and peroxisome proliferator activated receptors (PPARa and ). The hypothesis to be tested is that the actions of various nuclear receptors are orchestrated by RA-RXRa and that these nuclear receptors, in turn, directly or indirectly regulate pathways that are important for hepatocyte proliferation. Four specific aims are proposed to test this theory. The experiments in Aim 1 will utilize nuclear receptor knockout mice to assess the role of specific heterodimeric RXRa partners on hepatocyte proliferation during liver regeneration. The studies in Aim 2 are designed to define the mechanism by which nuclear receptor Nur77 regulates hepatocyte proliferation. To this end, experiments will be performed to study the induction, intracellular localization, and interaction between Nur77 and RXRa in directing this response. In Aim 3, we propose to study PPAR-mediated circadian cell cycle gene regulation and other pathways that dictate hepatocyte proliferation during liver regeneration. This will include analysis of whether PPARa and have redundant or unique roles in regulating pathways that control cell proliferation. Aim 4 entails a study of the effect of priming RA isomers in activation of specific nuclear receptors and the downstream pathways to facilitate hepatocyte proliferation. As the arrest of hepatocyte proliferation is the major cause of liver failure, and deregulated hepatocyte proliferation leads to cancer, by providing new insights into these processes the proposed experiments could result in the design of novel therapeutics to manage these conditions. These data could also help improve the way retinoids are used to prevent and treat certain types of cancer. PUBLIC HEALTH RELEVANCE: The goal of this project is to delineate the action of retinoid-mediated signaling in regulating liver cell proliferation during liver regeneration. Appreciation of the mechanisms controlling hepatocyte proliferation should provide the means to enhance liver regeneration in response to liver injury, to manipulate the liver mass during liver transplantation, and to inhibit proliferation and treat liver cancer.